I've found that cuck's usage on 4chan dates back to even mid-2013, which is when I presume 4plebs' archive stops.
For the majority of 2013 cuck's usage seems to be in reference to the actual definition of the word so it shouldn't really be counted.
Cuck didn't seem to devolve into its modern definition of leftist/emasculated male until around December 2013.
Late November 2013 is when 4plebs stops archiving /pol/ data so this may not be entirely accurate. /tv/ goes on until April 2013 wherein the word was used by its actual definition and frequently as there had been (as there always is on /tv/) spam of cuckold porn posts on the board.
This spam of cuckold porn, prompting its mass-usage of the word probably catalyzed the word's devolution and subsequent memeification.
This is one of the earliest threads I found where the word was used by an OP to describe a leftist rather than a genuine cuckold, the OP also postulates on why leftists are inherently cuckolds so this may very well be the first usage of cuck in its modern definition.
http://archive.4plebs.org/pol/thread/23795577/#23795577